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Medical Histology and Virtual Microscopy Learning Resources

The University of Michigan Medical School

Digestive System 1: Oral Cavity and Salivary Glands

Atlas Wheater’s, Chapter 13, pgs 251-262, Oral tissues
Text Ross and Pawlina, Chapter 16, Digestive System I: Oral Cavity and Associated Structures
Lab Resources

OBJECTIVES:

  1. Know the location and histological similarities and dissimilarities among the different types of oral mucosae (lining, masticatory, specialized).
  2. Know the histology of the tongue including the different types of papillae.
  3. Know the histology of the adult tooth and name the cells responsible for the production of enamel, dentin, and cementum.
  4. Describe how the tooth is anchored in alveolar bone.
  5. Know the structure of the epithelial attachment and understand its significance in protecting the periodontal space.
  6. Be able to identify parotid, submandibular and sublingual salivary glands on the basis of histological appearance and by the types of secretion produced by each gland.
  7. Be able to identify striated ducts of the salivary gland at the light and electron microscope level and correlate the structural features of the constituent cells to the functions of these ducts.
  8. Know the localization of myoepithelial cells and nerves in relation to the acinar cells, and their role in secretory functions.

I. Oral Cavity (W 251-2)

The oral cavity is lined by a mucous membrane (the oral mucosa) consisting of a stratified squamous epithelium, which may or may not be keratinized, and an underlying connective tissue layer, the lamina propria. The surface is kept moist with mucus produced by the major and numerous minor salivary glands. The oral mucosa is well supplied with nerve endings and, on the dorsal surface of the tongue, special sensory endings for taste. The submucosa underlying the lamina propria of the oral cavity is variable. At times the lamina propria and submucosa are substantively so similar that they merge imperceptibly. In your slides, the submucosa will be distinguished from the lamina propria only by the presence of minor salivary glands in a loose textured tissue, and we do not ask you to recognize this distinction microscopically.

A. Oral Mucosa (Mucous Membrane)

1. Lining Mucosa
Slide 114R (lip, human, H&E) WebScope ImageScope
Slide 114 triC (lip, human, trichrome) WebScope ImageScope
Slide 114M (lip, monkey, H&E) WebScope ImageScope

A stratified squamous non-keratinized epithelium lines the oral surface of the lips, cheeks, floor of mouth, and covers the ventral surface of the tongue In slide 114 (human) and 114M (monkey) of the lip, note that skin (stratified, keratinized squamous epithelium with hair follicles) covers the external surface [example], skeletal muscle (orbicularis oris muscle) forms the core [example], and a mucosal epithelium (stratified, non-keratinizing squamous epithelium) covers the internal surface [example]. A lamina propria underlies the mucosa and small salivary glands  (labial salivary glands) [example] are present in the submucosa. Note the transition zone between the keratinized epithelium of the skin and the nonkeratinized epithelium of the mucosa. This transition zone is called the vermillion zone (present only in humans) [example]. In the transition zone, long connective tissue papillae extend deep into the epithelium. Capillaries are carried close to the surface in these papillae. Because the epithelium is very thin in this region, the lips appear red (this arrangement may or may not be apparent in your glass slides). Salivary glands are lacking in the vermillion zone, therefore, the lips must be continuously moistened (by the tongue) to prevent drying out.

2. Masticatory Mucosa
Slide 115 (fetal palate, H&E) WebScope ImageScope
Slide 115 (fetal palate, trichrome) WebScope ImageScope

A stratified squamous keratinized epithelium is found on surfaces subject to the abrasion that occurs with mastication, e.g., the roof of the mouth (palate; W pg 252, 13.2) and gums (gingiva; W pg 257,13.9). Slide 115, which you used to study bone and the respiratory system, is a longitudinal section through the palate and includes the lip, gingiva, hard palate, and a portion of the soft palate [ORIENTATION]. This tissue is from a term fetus (with unerrupted teeth) and the epithelium over the hard palate is not yet fully differentiated (i.e. not fully keratinized). The slide is, however, a good overall orientation to the histology of the hard and soft palate. In the adult the epithelium of the hard palate is keratinized. Identify respiratory epithelium, bone (hard palate), forming tooth [example], and skeletal muscle in the lip [example] and the soft palate [example]. Some slides show mucous salivary glands [example] in the submucosa.

3. Specialized Mucosa
Slide 116 20x (tongue, H&E) WebScope ImageScope
Slide 116 40x (tongue, H&E) WebScope ImageScope
Slide 117 20x (tongue, H&E) WebScope ImageScope
Slide 117 40x (tongue, H&E) WebScope ImageScope
Slide 117N 40x (tongue, rabbit, H&E) WebScope ImageScope

The dorsal surface and lateral borders of the tongue (W pgs 258-9, 13.10-12) are covered by a mucous membrane that contains nerve endings for general sensory reception and taste perception. In slide 116, the dorsal surface of the tongue is covered with tiny projections called papillae [example], which are lacking on the ventral surface. The body of the tongue is composed of interlacing bundles of skeletal muscle [example] that cross one another at right angles. The dense lamina propria of the mucosa is continuous with the connective tissue of the muscle, tightly binding the mucous membrane to the muscle. Some glass slides in our collection show mucous glands in the submucosa, which are found only on the ventral side of the tongue. These glands are not present in the digital slides, but their ducts may be seen [example].

In slide 116 there are two types of papillae on the tongue (W pg 258, 13.10-11). Locate the numerous filiform papillae [example], that appear as conical structures with a core of lamina propria covered by a keratinized epithelium. Fungiform papillae [example] are scattered among the filiform papillae. They have expanded smooth round tops and narrower bases. In young children, the fungiform papillae can be seen with the naked eye as red spots on the dorsum of the tongue (because the non-keratinized epithelium is relatively translucent). These papillae are less readily observed in adults, because of slight keratinization of the epithelium.

Slide 117 and especially slide 117N contain examples of circumvallate papillae (W pg 259, 13.12) [example]. These are large circular papillae surrounded by a deep trench. The covering epithelium is non-keratinized. Taste buds (W pg 400, 21.1) [example], the chemoreceptors for the sense of taste, are located on the lateral borders. Each taste bud contains about 50 spindle shaped cells that are classically described based on their appearance as “light” (receptor) cells, “dark” (supporting) cells, and “basal” (stem) cells, although these distinctions are difficult to see in your slides so we do not require you to identify the cell types. Non-myelinated nerves from cranial nerves VII, IX, or X (depending on the location of the taste bud) synapse with the receptor and, to some extent, supporting cells of the taste bud. Some slides show serous glands (of von Ebner) [example] in the lamina propria and interspersed between the bundles of muscle beneath the papillae. These glands drain into the base of the trench around the circumvallate papillae.

B. Gingiva and Tooth in Jaw (W pg 252, 13.3)
Slide 122 40x (tooth, monkey, H&E) WebScope ImageScope
Slide 122 20x (tooth, monkey, trichrome) WebScope ImageScope
Slide 122PAS 20x (tooth, monkey, PAS) WebScope ImageScope
Slide 123 40x (tooth, pig, H&E) WebScope ImageScope
Note: due to technical diffuculties, the quality of these virtual slides is less than ideal; it is highly recommended that you use your glass slides and microscopes to study this material.

1. Gingiva

There are three different stains: H&E, PAS & H, and Trichrome. These slides need some understanding of sectioning a three dimensional object in two dimensions! See the section covering the histology of the oral cavity in the MMH-CD if you need help. There is reasonable, undistorted tissue in some regions of these sections. Be sure you look at all three. The gingiva is the highly keratinized epithelium and underlying connective tissue lamina propria that surrounds the teeth. In this slide, the epithelium is completely keratinized. The extensive interdigitation with the lamina propria increases the surface area for epithelial attachment to the connective tissue. On one side of the section you can trace the transition from keratinized gingiva to nonkeratinized mucosa (in the PAS slide, some glycogen is stained in the mucosa).

2. Tooth

The tooth in these sections lacks its outer layer of enamel because the tissue has been decalcified (recall that enamel is ~95% mineral); however, the matrix of the dentin is well preserved as is the cementum and adjacent matrix of the alveolar bone of the mandible. This is a multi-rooted tooth and very few slides exhibit the complete extent of the root. However, you should be able to see excellent regions of the periodontal ligament [example] connecting the root cementum to the alveolar bone (W pg 257, 13.8). If you use a light microscope and close down the iris diaphragm you can see Sharpey’s fibers inserting into both the alveolar bone and root cementum. Note the extensive vascular supply to the periodontal ligament that travels in (an interconnected network of) loose connective tissue.

Find the cemento-enamel junction [example] and trace the acellular cementum toward the root apex where there is a transition [example] to cellular cementum [ORIENTATION]. The cellular cementum may be identified by the presence of cementocytes which, much like osteocytes, reside within lacunae (you will need to use your microscopes and glass slides to see this well). This monkey tooth has tortuous roots and the cellular cementum is usually present in a tangential section. In most sections, a series of longitudinally oriented increments of bone line the alveolar socket (where periodontal ligament fibers insert). Alveolar bone appears more like immature, or woven, bone and can be distinguished from the Haversian systems deeper within the bone, is particularly labile and is easily resorbed and deposited as a consequence of tooth movement. The dentinal tubules [example] can be seen in the dentin (W pg 256, 13.6) and, in many sections, distorted odontoblasts line the pulp cavity (W pg 256, 13.7) which is otherwise filled with loose (almost mesenchymal) connective tissue (W pg 242, 13.6-7). Odontoblasts and dentinal tubules can be seen very well in slide 123, which is of a tooth that is just about to erupt --ameloblasts can also be seen although the enamel is lacking since this section was also decalcified during processing.

The region of greatest clinical importance is the site of gingival attachment to the tooth [example] --again, since the enamel has been removed, the actual attachment cannot be seen, but you should be able to discern the approximate area where this occurs. This epithelial attachment (W pg 257, 13.9) to the enamel surface can break down leading to a chronic infection (periodontal disease). If the disease persists, the attachment epithelium and subjacent connective tissue become heavily infiltrated with white cells (you won’t see this in your slides as the tissue was healthy when the slides were prepared –you may see a few WBCs, but this is normal). The disease can progress resulting in loss of alveolar bone, periodontal ligament and, eventually, the tooth. Infections in this region can be important in patients with AIDS, patients receiving chemotherapy or radiation for cancer, and patients that have had a history of streptococcal and/or staphylococcal infections involving the heart, among others.

II. SALIVARY GLANDS (W pgs 246-8)

We will study three major salivary glands; the parotid, submandibular, and sublingual glands.  These glands are divided into numerous lobules which contain the secretory units, which are called acini.  Each secretory unit consists of acinar (secretory) cells and branching ducts.  Ducts within lobules (intralobular ducts) connect with larger interlobular ducts which are found in the connective tissue between lobules.

A. Parotid gland (W pgs 260-1, 13.15, 13.18)

Slide 180-1 20x WebScope ImageScope
Slide 180-1 40x WebScope ImageScope
Slide 180-2 20x WebScope ImageScope
Slide 180-2 40x WebScope ImageScope

At low magnification, observe the abundance of fat in this gland: the amount of fat increases with age. Note that the parenchymal (secretory) tissue is divided into many lobules by the stromal (supportive) connective tissue. In this connective tissue stroma, notice the presence of blood vessels, nerves and large, interlobular ducts [example]. These interlobular ducts are lined by a pseudostratified columnar epithelium. Find a well defined lobule [example] and observe the secretory acinar cells and ducts within each lobule. You will note that these acinar cells are fairly uniform in size, structure and staining, because they all produce proteinaceous, or serous, secretion. The secretory granules are not always well preserved by routine methods of light microscopic fixation and are not recognizable in this slide.

The ducts of various sizes present within a lobule (intralobular ducts) are lined by a simple columnar epithelium of varying heights. Select a transverse section of a medium-sized duct and observe the fine striations at the basal portions of the lining cells (W pg 262, 13.19). Ducts with these striations are the striated ducts (or salivary ducts) [example]. What are the "striations" of striated ducts? (DG1) The slender, intercalated ducts (W pg 261, 13.18) [example] are lined by low-cuboidal epithelial cells and connect the secretory acini and striated ducts, but the intercalated ducts in this gland are much shorter than those in the pancreas (which we will cover in another session), so they are not as easy to spot.

B. Submandibular and Sublingual Glands (W pgs. 261-2, 13.16, 17, 20)
Submandibular Gland:
Slide 183-1 20x WebScope ImageScope
Slide 183-1 40x WebScope ImageScope
Slide 183-2 (mucicarmine stain) WebScope ImageScope
Slide 184 20x WebScope ImageScope
Slide 184 40x WebScope ImageScope
Slide 184-2 40x WebScope ImageScope

Sublingual Gland:
Slide 185 20x WebScope ImageScope
Slide 185-1 40x WebScope ImageScope
Slide 185-2 40x WebScope ImageScope
Slide 185A (mucicarmine stain) WebScope ImageScope

Unlike the parotid gland, the submandibular and sublingual glands possess both mucous and serous secretory cells. Slides 183-2 (submandibular) and 185A (sublingual) are stained with mucicarmine which specifically stains mucus red. Survey the two alternate slides to compare the relative proportions of mucous acini in these two glands. After this, go back to the H&E-stained slides to study the histology of mucous and serous secretory acini.

In slide 183 and 184 of the submandibular gland, mucous secretory acini are those that stain lightly. Compare the appearance of mucous and serous secretory cells (W pg 261, 13.17). The serous cells possess granular cytoplasm and nuclei which are spherical and vesicular in appearance. The mucous cells have pale staining cytoplasm and nuclei which appear to be pushed against the basal cell membrane. Now, observe the blind end of a mucous acinus and note the serous demilune cells [example] that cap this region of the secretory acinus. These demilune cells also have brightly staining granules in their cytoplasm. As an aside, the serous and mucous cells are actually adjacent to each other in vivo, and the formation of these demilunes is actually a fixation artifact (mucous cells swell with traditional fixation techniques and “squeeze out” the serous cells; see Ross pg 496 and figs 16.23 and 26). Even so, the characteristic appearance of demilunes is nonetheless a useful diagnostic for identifying sero-mucous glands. Switch to a lower power and observe the distribution of ducts. Note the presence of large ducts in the connective tissue septae separating the lobules. These are interlobular or excretory ducts [example] and their epithelium appears to be stratified. The intralobular ducts within the lobules are smaller and similar in appearance to those of the parotid gland (i.e. they are striated ducts). The intercalated ducts in the submandibular and sublingual glands are very short and therefore NOT encountered often in sections; for this reason, you will NOT be required to identify intercalated ducts in the submandibular or sublingual glands.
           
Now examine slide 185, the sublingual gland, and note the greater prominence of mucous acini and fewer numbers of intralobular ducts than in the submandibular gland (W pg 261, 13.16). Some lobules in slide 185 reveal almost all ducts with a few remaining secretory cells. This represents a pathological transformation of secretory acini to duct-like structures.

Electron Micrograph Wall Charts

#94 ORAL EPITHELIUM (NON-KERATINIZED) WebScope ImageScope

The basal lamina (basement membrane) is not seen in this view, but the lower cells of this epithelium do represent basal cells. Note the change in cell shape as the cells move from basal to superficial layers. Nuclei are present in the surface cells.

#95 TASTE BUD WebScope ImageScope

Note the difference in cell shape and cell aggregation of the taste bud and the oral epithelium. The nerve endings are sensory nerves.

#99 LINGUAL (VON EBNER'S) GLAND WebScope ImageScope

The acinar lumen (serous) is always quite small and narrow, and is not always seen in electron micrographs. Find one here. The nuclei of serous secretory cells are usually round in contrast to those in mucous acini, which are often compressed and flattened against the base of the cell. Note the great abundance of secretory granules. The intralobular duct cells may, occasionally, also contain secretory granules. Find the myoepithelial cell of the duct wall. Try to follow the peripheral border of one serous acinus. The interacinar connective tissue is very thin and delicate.

#98 SUBLINGUAL GLAND WebScope ImageScope

Note the difference in number between the mucous acini and the serous demilunes.  Find the lumen of an acinus.  Observe that, as described above, the cells of the serous demilunes do not always border directly on the acinar lumen but instead are pushed to the outer edge of the acinus as an artifact of fixation.  As the serous cells are squeezed out, however, small extensions of the their cytoplasm (somewhat erroneously called “secretory canaliculi”) remain in contact with the acinar lumen.  Note that the intercalated ducts are small, narrow channels in comparison to interlobular and striated ducts (not seen in this micrograph).  Although not marked on the micrograph, can you find blood capillaries in the connective tissue spaces?

Review Questions

DG1: What are the "striations" of striated ducts? Answer

Practice Questions

  1. The epithelium specifically indicated is from: click here to see low magnification image
    click here to see medium magnification image

    1. vermillion zone of lip
    2. filliform papilla of tongue
    3. fungiform papilla of tongue
    4. circumvallate papilla of tongue
    5. mucosa of pharynx
    6. mucosa of esophagus
    7. Answer
  2. The periodontal ligament connects:
    1. dentin to enamel.
    2. dentin to cementum.
    3. cementum to alveolar bone.
    4. enamel to alveolar bone.
    5. the root of one tooth to the root of an adjacent tooth.
    6. Answer
  3. The structure indicated contains a: click here to see low magnification image ; click here to see medium magnification image ; click here to see high magnification image
    1. cell process of an ameleoblast
    2. cell process of an odontoblast
    3. cell process of a cementocyte
    4. Sharpey's fiber
    5. capillary from pulp cavity
    6. Answer
  4. The tissue shown is from: click here to see low magnification image ; click here to see medium magnification image ; click here to see high magnification image
    1. parotid gland
    2. sublingual gland
    3. submandibular gland
    4. von Ebner's gland
    5. labial (i.e. minor) salivary gland
    6. Answer